1. Field Of The Invention
The present invention relates generally to the manufacture of a communications cable having one or more communications elements disposed within a conduit. More particularly, the present invention relates to a method and apparatus for determining and controlling the excess length of a communications element, such as an optical fiber or optical fiber ribbon, contained within a conduit, such as a buffer tube.
2. Discussion of Related Art
In some types of optical fiber cables, optical fibers or optical fiber ribbons are disposed within a buffer tube. It is desirable for the length of such communications elements to be longer than the buffer tube to allow a tensile stress to be applied to the buffer tube without imposing stress on the optical fibers. If the optical fibers are not longer than the buffer tube and the cable is placed in tension, which can occur during installation and operation of the cable, the optical fibers can be subjected to a tensile stress and under severe conditions, the optical fibers may break due to the limited amount of strain such fibers can endure. In addition, an increase in the strain on optical fibers can cause attenuation. However, if an excess length of optical fibers is disposed within the buffer tube, application of tensile stress to the buffer tube may cause the fibers to straighten out, but the tensile stress and strain on the fibers is avoided.
The length of optical fiber or optical fiber ribbon which exceeds the length of the buffer tube is typically referred to as "Excess Fiber Length" or "Excess Ribbon Length" as the case may be. The terms "Excess Fiber Length" and "Excess Ribbon Length" are referred to herein collectively as the "Excess Length" or simply "EL." Cable manufacturers also refer to a ratio called "Excess Fiber Length Ratio" or "Excess Ribbon Length Ratio" as the case may be (referred to herein collectively as the "Excess Length Ratio" or "EL Ratio"), which is the number which results from the following equation: ##EQU1## where F.sub.L =Fiber length, and where T.sub.L =Tube length.
While Excess Length and an Excess Length Ratio greater than one (1) is desirable to prevent tensioning the fiber, in some cases the EL and the EL Ratio can be too high resulting in greater signal attenuation due to excess bending of the fiber. Also, as the EL and EL Ratio increases, the amount of fiber used increases, which increases the cost of the cable. Therefore, it is desirable to control the EL within a buffer tube by controlling the EL Ratio.
There are two known methods of determining the EL Ratio, the destructive testing method and the speed or velocity method. In the destructive testing method, the buffer tube and fiber or ribbon residing therein are cut to produce a sample, and the fiber or ribbon is removed from the buffer tube. The lengths of the fiber or ribbon and buffer tube are then measured, and the EL Ratio is calculated.
The destructive testing method has several disadvantages. First, the destructive testing method leads to waste because the sample is destroyed during analysis. Second, the destructive testing method does not provide a realistic means for calculating the EL Ratio at many different points on a buffer tube because the buffer tube is destroyed during testing in order to determine the EL Ratio. Finally, the destructive testing method does not provide a means for controlling the EL Ratio in real time or near real time during the manufacturing process because of the time required to cut and measure the sample.
In the velocity method for determining the EL Ratio, the rate at which the fiber is fed to a buffer tube extruder cross-head and the line velocity of the buffer tube extruded over the fiber are measured. The line velocity and feed rate correlate directly to the buffer tube length and fiber length respectively. The velocity method typically employs two non-contact laser length gauges as sensors to determine the fiber feed rate and buffer tube line velocity. The feed rate and line velocity information are entered into a pre-programmed computer to determine the EL Ratio.
The velocity method has the advantage over the destructive testing method in that there is no destruction of the buffer tube or cable. However, because the buffer tube may shrink or otherwise be altered by the manufacturing process, or subsequent to the manufacturing process, such that the EL Ratio is also altered, the velocity method can not provide a reliable post-extrusion process measure of the EL Ratio of the product. For example, it is well known that while on a reel either prior to a phase of manufacturing the cable or prior to installation of the cable, the buffer tube length may change due to additional post-extrusion shrinkage of the buffer tube material such that the EL Ratio is increased. Therefore, the velocity method does not provide a means for determining or controlling the EL Ratio after the extrusion process.